In order to prevent stray light from reaching an image plane and/or to make jumbled inner part of a lens barrel unseen from the outside, a light-shielding plate like an iris diaphragm is fixedly arranged inside the lens barrel.
Another embodiment of the stray light eliminating arrangement has been disclosed, which is a lens barrel that comprises a first lens frame member moving along the optical axis, a second lens frame member relatively moving closer to and apart from the first lens frame member, and a light-shielding member capable of blocking beams between the first and second lens frame members from leaking, the light-shielding member being sufficiently elastic to be transformed to have a segment of an inflection curve along the optical axis as the first and second lens frame members come close to each other (see Patent Document 1 listed below).
Still another embodiment of the stray light eliminating arrangement has also been disclosed, which is a lens barrel that comprises first and second groups of lens pieces relatively varied in an interval therebetween along the optical axis and that also comprises a light-shielding device held by the first group of lens pieces and/or the second group of lens pieces and consisting of a plurality of approximately cylindrical light-shielding members. Each of the light-shielding members partially overlaps an adjacent one in series along the optical axis, and an amount of the overlap of the adjacent ones of the light-shielding members is varied depending upon the interval between the groups of lens pieces, so as to permit the light-shielding device to telescopically expand and shrink (see Patent Document 2 listed below).
A zoom lens barrel using plastic cams has those cams collectively arranged in a single cam barrel member as much as possible in order to attain a control of movements of the groups of lens pieces. Specifically, the single plastic cam barrel is provided with convex cams, cam grooves, tapered cams, and the like. In order to avoid a light but sharp feeling of shock that the cam barrel makes while it is being revolved for the zooming or the like, cam barrel components located within a range of a stroke from a wide-angle view to a telephoto view are to be molded without seam or parting line that is likely to be caused along a junction of dies.
With the convex cams employed in the cam barrel, its surface areas with the cams are radially raised by one step from those without cams. If the convex cams extend along the fully encircling surface of the cam barrel, light cannot find a clearance to slip in or out between distinct convexities radially raised from the cam barrel, resulting in the stray light being avoided.
Actually, however, the convex cams can not be provided along the full girth of the cam barrel because the seam, namely, the parting line resulted from the junction of the dies should not exist in the surface areas with the convex cams for the purpose of the completely smooth stroke of the cam barrel as mentioned above. Thus, there is a clearance partially between the surface areas provided with the convex cams in the cam barrel and another barrel component provided with sliding studs that are engaged with the convex cams. Light beams, after passing through the clearance, are directly directed to an imaging plane of film or CCD, or first reflected from an inner surface of the lens barrel and then directed to the imaging plane of the same, causing the resultant image to be deteriorated in vividness.
In addition, when the opposite lateral walls defining each cam groove are not parallel to each other, with their respective outward portions spreading more apart from each other to have a trapezoidal vertical cross-section of the cam groove, the sliding studs have to be forced against the bottom of the cam grooves by using some pushing means such as a coil spring. To employ this means, it is inevitable to have the sliding studs greater in diameter and accordingly have the cam grooves larger in width, and as a consequence, the stray light is more prone to occur.
The lens barrel as disclosed in Patent Document 1 works effectively only if it has a configuration where the a distance between first and second groups of lens pieces is varied only slightly as a result of the stroke of the zoom lens from the telephoto view to the wide-angle view or vice versa. In order to permit the distance between the first and second groups of lens pieces to be varied more greatly, the elastic light-shielding member must be greater in dimensions, which makes the process of attaching the light-shielding member laborious and unstable. Additionally, the elastic light-shielding member is transformed more as a result of the stroke of the zoom lens, and the elastic light-shielding member transformed to a greater extent is likely to lose its ability to shield light as it can under normal conditions.
The lens barrel as disclosed in Patent Document 2 has a disadvantage that the plurality of the conical and approximately cylindrical members, as a result of having any adjacent couple of them partially overlapped with each other to serve as the light-shielding device, require the lens barrel increased in diameter to house them. Since the combined conical cylindrical members have their collective extension along the optical axis restricted to the minimum of the distance between the first and second groups of lens pieces and are limited in number depending upon the maximum of the distance, the lens barrel is more likely to have a greater diameter when the distance between the first and second groups of lens pieces is varied more as a result of the stroke of the zoom lens.
<List of Patent Documents>
    Patent Document 1:            Japanese Patent Publication of Unexamined Application No. H11-84198            Patent Document 2:            Japanese Patent Publication of Unexamined Application No. 2003-66315        
Accordingly, it is an object of the present invention to improve the prior art lens systems, especially, the prior art zoom lens systems where groups of lens pieces move within a lens barrel for the zooming and the focusing and to provide an improved zoom lens system that is simplified and robust in configuration so as to withstand malfunctions well and that is capable of effectively and assuredly eliminating stray light.
A zoom lens system according to the present invention comprises a first cam barrel rotating about the optical axis and a second cam barrel located inside the first cam barrel in cam-linkage with the same, and the zoom lens system is characterized in that the second cam barrel has openings to let light beams pass through, and that a light-shielding plate like an iris diaphragm is supported by the second cam barrel in position closer to the photographed subject in the second cam barrel so as to shield the light beams that come in from the openings in the second cam barrel after going out of the openings in the second cam barrel and then being reflected from an inner surface of the first cam barrel.
In an aspect of the present invention, the openings in the second cam barrel are clearances defined to avoid an interference of the second cam barrel with a driving member used to move components within the second cam barrel.
Convex cams are provided in an inner surface of the first cam barrel or an outer surface of the second cam barrel, and the light beams that are to be reflected from the inner surface of the first cam barrel pass through portions where the convex cams are terminated.
A zoom lens system, which has groups of lens pieces moved within a lens barrel for the zooming and the focusing according to the present invention, is simplified and robust in configuration so as to withstand malfunctions well and is capable of effectively and assuredly eliminating stray light.